1. Field of the Invention
The present invention is directed to a magnetic resonance installation of the type having magnet systems with which a basic magnetic field, gradient magnetic fields and a high-frequency magnetic field can be generated, and having a start element that is connected to an end element via a coaxial cable, the coaxial cable having an inner conductor and an outer conductor.
2. Description of the Prior Art
A magnetic resonance installation of the above type is disclosed, for example, by European Application 0 337 204, corresponding to U.S. Pat. No. 4,922,204.
A current may be induced in the outer conductors in the coaxial cables due to the variable high-frequency magnetic fields. Such currents are suppressed with traps.
In the trap known from European Application 0 337 204 the coaxial cable is wound as a toroid and a capacitor is connected in parallel with it. The toroid, which thus forms an inductor, is tuned with the capacitor to the basic frequency of the magnetic resonance installation. As a result, the high-frequency current on the outer conductor of the coaxial cable is interrupted.
The known trap is relatively narrow-band, and therefore must be set very exactly. The exact setting is often very difficult, particularly because of the intrinsic capacitance of the coaxial cable. Further, it must be assured that the frequency that has been set does not drift over time.
An object of the present invention is to provide a simple, reliable and economic arrangement that forms a broadband trap in a magnetic resonance installation.
This object is achieved in accordance with the invention in a magnetic resonance apparatus of the type initially described wherein the coaxial cable has at least two cable sections; that are coupled to one another via a transformer with transformer coils; with the transformer coils being directly connected to the inner conductors of the respective sections and being connected to the outer conductors of the respective sections via tuning capacitors.
When the high-frequency magnetic field exhibits a basic frequency and the transformer coils and the tuning capacitors form frequency filters tuned to the basic frequency, the trap is especially effective.
When the inner conductors and the outer conductors are galvanically connected to one another via high-frequency inductances, a signal transmission via the coaxial cable is also additionally possible in the low-frequency range, i.e. at frequencies considerably lower than the basic frequency.
When the high-frequency inductances galvanically connecting the inside and the outer conductors to one another have a middle region and two outside regions and the outside regions are connected to the outer conductors via auxiliary capacitors, the galvanic connection of outer conductor and inner conductor also contains a trap.
When the outside regions and the auxiliary capacitors form low-pass filters with a limit frequency that is lower then the basic frequency, this additional trap is also especially effective.
Low-frequency signals that the invention allows to be transmitted via the signal line are control signals that can be communicated between the start element and the end element via the coaxial cable.
The inventive arrangement is preferably utilized when the start element is a control and evaluation unit for the magnetic resonance installation and the end element is a local coil, because the coaxial cable is necessarily exposed to the high-frequency field in this case.